SAN DIEGO (AP) -- The Marine Corps is forming the first squadron of pilots to fly the next-generation strike fighter jet, months after lawmakers raised concern that there was a rush to end the testing of the aircraft that has had technical problems.

So far, two veteran pilots of the 3rd Marine Aircraft Wing have been trained to fly the F-35B. They are becoming the first members of Marine Fighter Attack Squadron 121 that will debut today at a ceremony at the Marine Corps Air Station in Yuma, Ariz.

The first F-35B arrived Friday, and 15 more are coming during the next year. The Defense Department has pumped a half a billion dollars into upgrading the facilities, hangars and runways at the base to make way for the supersonic fighter jet named Lightning II, officials said.

The squadron's pilots are expected to fly the aircraft by year's end.

The Marines are the first in the military taking the steps toward putting the planes in operation. The F-35B would replace Cold War-era aircraft such as the F/A-18 Hornet and AV-8B Harrier.

"It's a pretty big milestone that a lot of people are looking at and judging," said Marine Capt. Staci Reidinger, a spokeswoman at the Yuma base. "The lessons learned will be shared."

Today's inauguration comes only months after leaders of the Senate Armed Services Committee suggested that Defense Secretary Leon Panetta rushed a decision to develop the Marine Corps version of the fighter jet.

In a letter sent in February to the Pentagon chief, Sens. Carl Levin, the committee chairman, and John McCain, the panel's top Republican, questioned whether the F-35B had met the criteria to warrant an end to its probation. The F-35B had been on a two-year probation because of "significant testing problems."

Levin, D-Mich., and McCain, R-Ariz., wrote that the program "has enjoyed some success over the last few months, after several years of having fallen short." But they said "more problems with the F-35B's structure and propulsion, potentially as serious as those that were originally identified a year ago, have been found. This is salient where the F-35B has completed only 20 percent of its developmental test plan to date. Your decision, therefore, appears at least premature."

Levin declined to comment on the squadron. McCain did not respond to a request from The Associated Press for comment.

The developer of the aircraft, Lockheed Martin Aeronautics Co., is building three versions of the F-35 -- one each for the Navy, Air Force and Marine Corps and some foreign forces.

Schedule delays and cost overruns have dogged the F-35's development, making it the Pentagon's most expensive weapons program ever. Ten years in, the total F-35 program cost has jumped from $233 billion to an estimated $385 billion. Recent estimates suggest the entire program could exceed $1 trillion over 50 years.

Aviation safety consultant and retired Marine Corps Col. Pete Field, who served as the former director of the Naval Test Pilot School, said the Marine Corps' F-35B is the most complicated of the three versions because it can take off and land vertically in less than 500 feet of space, allowing it to be launched from small Navy ships and to drop down in confined areas.

One of the problems earlier on in its development was it was heavier than predicted, which would have compromised its unique ability to take off and land vertically, Field said.

It was lightened substantially, but Field said that could also mean structural problems in the long run.

"It remains to be seen if the F-35B has a long life and is structurally sound," said Field, who was the chief test pilot for the F/A-18. "We won't know for two to three years after it's been in operation. If nothing crops up, perhaps the engineers have done their best work."

Its sophisticated stealth capabilities mean that like the Air Force's F-22, the aircraft can fly into enemy territory without being detected by radar.

"All we can do is hope that they have solved all the program problems and that they've got a pretty good airplane," Field said.

Officials at Lockheed Martin Corp. say they have corrected the problems with the aircraft's structure and propulsion in the development phase. The fixes addressed cracking in the interior bulkhead and the inadequate fitting of doors atop the plane that open to allow extra air to reach the engines, among other issues.

There were no accidents or close calls by pilots testing the F-35B, said company spokesman Jack Giese.

The commanding officer of the new squadron, Lt. Col. Jeffrey B. Scott, said the F-35B will give the nation a "giant leap" in its strategic capabilities by being able to drop in quickly to remote trouble spots without the need of having to be launched from a big deck Navy ship. Since the F-35 variants share similarities, the aircraft also will unify the military's air power.

"It flies extremely well. It's got a lot of great capabilities and it is growing into those capabilities," he said, adding later: "If you look at the history of any aircraft during development, this has had no more problems than any other previous aircraft, and we have the engineering solutions."